SDC: Don't perform natural propagation

Alias nets in clock targets cause errors in VPR

Signed-off-by: Tomasz Michalak <tmichalak@antmicro.com>

sdc-plugin/sdc.cc

diff --git a/sdc-plugin/sdc.cc b/sdc-plugin/sdc.cc
index 73eec15..888fbdf 100644
--- a/sdc-plugin/sdc.cc
+++ b/sdc-plugin/sdc.cc
@@ -229,9 +229,7 @@
 	    log_cmd_error("No top module selected\n");
 	}
 
-	std::array<std::unique_ptr<Propagation>, 3> passes{
-	    std::unique_ptr<NaturalPropagation>(
-	        new NaturalPropagation(design, this)),
+	std::array<std::unique_ptr<Propagation>, 2> passes{
 	    std::unique_ptr<BufferPropagation>(
 	        new BufferPropagation(design, this)),
 	    std::unique_ptr<ClockDividerPropagation>(